Remarks The classification of the articulate crinoids used herein follows Hess and Messing (2011).
Class Crinoidea J.S. Miller, 1821
Subclass Articulata von Zittel, 1879
Order Isocrinida Sieverts-Doreck, 1952
Suborder Isocrinina Sieverts-Doreck, 1952
Family Isselicrinidae Klikushin, 1977
Subfamily Isselicrininae Klikushin, 1977
Genus Isselicrinus Rovereto, 1914
Type species Isselicrinus insculptus Rovereto, 1914, p. 177 (= ?Pentacrinus didactylus d’Orbigny in d’Archiac, 1846), by monotypy, from the Oligocene of Ligurien, coastal north-west Italy (Biese and Sieverts-Doreck 1971; Jagt 1999, p. 74; Hess in Hess and Messing 2011, p. 61).
Diagnosis of stem (After Hess in Hess and Messing 2011, p. 63.) “Column cylindrical, pentalobate, or pentagonal to stellate in cross section. Columnal facets in most species similar to Balanocrinus, with uniform marginal crenulae and with adradial ridges or ribbons of minor crenulae or granules. However, some specimens, especially with small or pentalobate columnals, have a more or less gradual transition from marginal to adradial crenulae similar to Isocrinus, although petals are generally wide, drop shaped and almost subtriangular. Cirrus sockets rather small, facing downward from lower edge of nodal. Cirri short and slender”.
Remarks The stated similarity of some columnals to Isocrinus is of note. Isocrinus sensu stricto is restricted to the Mesozoic (Hess in Hess and Messing 2011, p. 50), yet columnals of similar morphology have been reported from the Antilles (e.g., Donovan and Veltkamp 2001, Middle Miocene Grand Bay Formation of Carriacou; Donovan et al. 2015, Upper Oligocene Antigua Formation of Antigua). Further comparisons may demonstrate these to be Isselicrinus, albeit distinct from those described herein (see below).
Klikushin (1977) subdivided the genus Isselicrinus into three subgenera, Isselicrinus Isselicrinus Rovereto, I. Praeisselicrinus Klikushin and I. Buchicrinus Klikushin. Praeisselicrinus was subsequently elevated to generic status, but Buchicrinus is not currently distinguished from Isselicrinus (Hess in Hess and Messing, 2011, pp. 61–65).
Range Upper Cretaceous (Maastrichtian) to Neogene (Miocene) (Hess in Hess and Messing, 2011, p. 63).
Isselicrinus sp. A
Material Six figured pluricolumnals, RGM.1332404–1332409 (Fig. 5); a further 14 specimens, RGM.1332410, are too fragmentary to be illustrated.
Locality and horizon Coastal bluffs at the northwestern corner of Bahía Inutil, upper Eocene, likely Moritzian Stage (Natland and Gonzalez 1974), Tierra del Fuego (Figs. 3, 4).
Description Heteromorphic crinoid pluricolumnals. Columnals vary from pentalobate (Fig. 5a–g) to rounded pentagonal in outline (Fig. 5h, i) and are all moderately large in diameter (c. 10 mm). Lumen is central, small, circular, surrounded by a low, circular perilumen. Areola comprised of five deeply sunken and teardrop-shaped petaloid pits in angles of the articular facet. Each areola petal is completely surrounded by a crenularium of short to peg-like crenulae; shorter, slender crenulae are rarer and situated between those extending for the full width. Each petaloid lobe is separated from each of its neighbours by a shallow groove gently widening toward the circumference except where truncated by a cirrus scar in nodals.
Three cirrus scars at the edge of the best preserved, proximal nodal facet (Fig. 5a). Cirrus scars are small, triangular, angled away from the facet and the latus. Cirrus lumen small, central and flanked by short fulcral ridges which appears serrated in one example. Cirrus facet depressed on either side of fulcrum.
Pluricolumnals vary from strongly pentalobate with convex latera to rounded pentagonal with planar latera. Latera smooth and unsculptured. Pentalobate pluricolumnals have a central suture(?) or groove along the length of each face of the latus; rounded pentagonal pluricolumnals lack such a groove, but there is a line of indentations, each corresponding to a suture between columnals. Pentalobate pluricolumnals have at least three orders of internodals and the nodal in the noditaxes, including at least 15 columnals, but internodals appear to be arrayed irregularly (Fig. 5d). In contrast, rounded pentagonal pluricolumnals have a heteromorphy based on seven internodals, N3231323.
Remarks In this small collection of pluricolumnals, there are two gross morphologies of pluricolumnal, the pentalobate (Fig. 5a–g) and the rounded pentagonal (Fig. 5h, i); all have a similar architecture to their articular facets. These are likely derived from the proximal and distal parts of the column, respectively, of a single species by comparison with other isocrinines (see, for example, Roux 1977; Donovan 1984).
Isselicrinus sp. B
? 2005 Isselicrinus sp.; Malumián and Olivero; fig. 3.
Material Six specimens, all UF: UF 231456 (two pluricolumnals; Fig. 6a–c, f); 243639 (long pluricolumnal; Fig. 6e, i); 243648 (pluricolumnal; Fig. 6g); 243689 (columnal; Fig. 6d); and 243693 (pluricolumnal; Fig. 6h).
Locality and horizon Northwestern side of Ponce; road cut on the southwestern side of PR-9, about 1.1 km east of junction with PR-123 (Fig. 1). Thinly bedded limestone; lower Oligocene Juana Diaz Formation.
Description Articular facets depressed centrally (Fig. 6c), although this may be an artefact of corrasion (compare with Malumián and Olivero 2005, fig. 3), or not (Fig. 6b, d, e). Lumen small, central, circular, surrounded by a low, rounded perilumen. Areola petals are teardrop-shaped to broader and more triangular, either strongly depressed (Fig. 6b, c) or less so (Fig. 6d), becoming near-planar (Fig. 6e). Crenularium well-developed at circumference, not extending all the way to the perilumen. Crenulae short, unbranched, approximately perpendicular to the circumference of the petals. Petals separated by shallow, radial grooves.
Pluricolumnals typified by long noditaxes (Fig. 6g–i). Figure 6c shows the proximal articular facet of a nodal bearing to cirrus scars angled away at c. 45° (at 1 and 6 o’clock as illustrated), possibly with the most proximal cirral ossicles still articulated. Cirral canal small, central.
The shortest pluricolumnal (Fig. 6f) is an incomplete noditaxis with unsculptured, slightly convex latera; the second specimen UF 231456, of similar form, is longer, comprised of 12 or more columnals and an incomplete(?) internoditaxis. In the centre of all sides there are circular depressions corresponding to plate sutures, corresponding to the grooves on the articular facets that separate the petals. The patterns of insertions of internodals appear ill-defined.
The longest pluricolumnal (Fig. 6i) is slender with a nodal ossicle at about mid-height, bearing an incomplete cirrus and a further cirrus scar. The cirrus scar is small, angled to the long axis of the pluricolumnal and circular in outline. The cirrus canal is small and central, flanked by a fulcral ridge. Cirrus ossicles are small, rounded and tapering away from the pluricolumnal. Other pluricolumnals have cirrus-bearing nodals at the centre (Fig. 6g, h); the latter pluricolumnal (UF 243693) has the longest preserved cirrus, albeit incomplete.
Remarks The specimens in Fig. 6 may appear to have been ‘lumped’ together. Although not identical to Isselicrinus sp. A, at least some of the specimens have a similar articular facet (compare Fig. 6b, c with Fig. 5a–c). But the smaller articular facets (Fig. 6d, e) of Isselicrinus sp. B are markedly different from the larger facets included in the same species. The small facets are interpreted as having been derived from a different part of the column, possibly more distal. This is supported, in part, by the similarities of all the pluricolumnals included in Isselicrinus sp. B, with nodals only bearing two cirri and external geometry broadly similar.
A notable feature of three of the five pluricolumnals is that they have not broken into noditaxes, but instead have nodals in the middle (Fig. 6g–i). That is, they are still articulated at the cryptosymplectial articulations at the distal facet of the nodals, considered the preferred site of initial autotomy in the isocrinine stem (see, for example, Donovan 1984; Hess in Hess and Messing 2011, p. 4). It is probable that these are broken fragments of longer pluricolumnals that had disarticulated at the distal cryptosymplectial articulations of nodal ossicles, but which were broken mechanically after they were exhumed by natural processes shortly before they were collected.